1. Field of the Invention
This invention relates to a ceramic arc tube of metal vapour discharge lamps such as high-pressure metal vapour discharge lamps and a method of producing the same. More particularly, the invention relates to a ceramic arc tube of discharge lamps which arc tube has an arc discharge portion integrally formed with electrode-holding end portions, the arc discharge portion having a larger outside diameter than that of the end portions, and a method of producing such ceramic arc tube.
2. Description of the Prior Art
High pressure metal vapour discharge lamps using recently-developed translucent polycrystalline alumina ceramic arc tubes, which arc tubes withstand vapours of such metals as sodium or metal halides, have a high luminous efficiency, so that such discharge lamps have attracted much attention from the standpoint of energy saving. In the description of the invention, the metal vapour discharge lamp refers to the high pressure sodium vapour discharge lamp, the metal halide vapour discharge lamp, or the like.
The metal vapour discharge lamp comprises an arc tube holding metal vapour and a protective envelope surrounding the arc tube. Accordingly, the arc tube is required to have both a good translucency of light and a high corrosion resistivity against the light-emitting material sealed therein such as sodium vapour or the metal halide vapour. Only translucent alumina ceramics has been found to meet the need of high corrosion resistivity against the light-emitting material and the good translucency, so that the alumina ceramics has been used almost exclusively for the arc tube of the high-pressure metal vapour discharge lamps.
The transluscent alumina ceramics, however, has a lower thermal malleability than the quartz. Thus, although the quartz arc tube for mercury-vapour lamps can be melted and sealed simply by heating it to a high temperature, the sealing of the alumina ceramics arc tube with the light-emitting material disposed therein requires a comparatively complicated process.
In a typical conventional process of sealing a translucent alumina ceramic arc tube, the opening ends of the fired alumina arc tube is sealed by means of glass frit material with the mounting caps made of either a heat-resistant metal or alumina ceramic which have a coefficient of thermal expansion similar to that of the alumina arc tube.
Furthermore, heat-resistant metallic electrodes provided with the through-holes for introducing the light emitting materials are sealed at the center portion of the above caps by glass frit.
The conventional sealing process has shortcomings in that the process is difficult to carry out because of the requirements of heating at the high temperature of 1,300.degree. to 1,400.degree. C. and in vacuo.
Moreover, in the glass frit sealed arc tube, the light emitting materials enclosed in the ceramic tube is susceptible to leakage due to the comparatively widely sealing area of glass frit, exposure to the high operating temperature and thermal shock caused by on-off operations of the lamp.
Especially, when being used in the improved discharge lamp provided with a high luminous efficiency and high colour rendering, the alumina tube sometimes fail to meet the required reliability including the errosion resistivity at a high temperature under high pressure. Furthermore, the use of the caps made of metal or ceramics results in an increased number of parts and requirement of severe dimensional accuracy, whereby the manufacturing cost becomes high and the products tend to be uneconomical.
To obviate the aforesaid shortcomings, the so-called semi-closed type alumina arc tubes have been proposed, in which ceramic caps are applied to opposite ends of each alumina tube before firing in such a manner that the caps are integrally secured to the alumina tube when they are fired together. More specifically, such semi-closed type alumina arc tube is generally produced by a method comprising steps of preparing a tubular green body having opposite ends thereof open by using an alumina series material whose firing shrinkage is fully known, preparing cap green bodies by using an alumina series material whose firing shrinkage is smaller than that of said tubular green body, fitting the cap green bodies in end openings of the tubular green body, and firing the tubular green body having the cap green bodies in vacuo or in hydrogen atmosphere. Whereby, a translucent alumina arc tube with caps integrally secured thereto is produced by the firing. This method of making the semi-closed type alumina arc tube has shortcomings in that the step of applying the cap green bodies to the tubular green body tends to cause deformation and damage of the green bodies, that the control of the firing shrinkages of the tubular green body and the cap green bodies is difficult, and that cracks are sometimes occured at end portions of the alumina arc tube to cause incomplete joint of the caps with the alumina arc tube which lead to possible leakage of the sealed light-emitting material.
In another method of the prior art to produce an arc tube having an alumina tube with caps integrally formed therewith by using the same material for the tube and the caps, a molding core made of a metal or organic substance having a low melting point is placed in the cavity of a die, and an integral body of the alumina tube with caps is formed in the space between the inner surface of the die and the molding core by applying pressure from the outside of the die. The molding core is then melted away by heating, out of the alumina arc tube. This method of using the molding core has technical difficulties in that pressing of the tubular alumina green body to the molding core tends to contaminate the tubular alumina green body with the material of the molding core, that the molten material of the molding core sometimes permeates into the alumina arc tube, and that traces of the molding core material left on the alumina arc tube become defects. Accordingly, this method of using the molding core has not commercially been used in industries due to the aforesaid technical difficulties.
The shape of the alumina arc tube for metal vapour discharge lamps has been limited to straight tube, because the malleability of the alumina arc tube is not so high as the quartz arc valve used in mercury-vapour lamps. The quartz valve can be easily shaped simply by heating it to a high temperature. Although the salient feature of the metal vapour discharge lamp depends on the high luminous efficiency, it is hard to further improve the luminous efficiency if the shape of the arc tube is limited to straight one. More specifically, the transmittance of the translucent alumina ceramics has been improved to 94 to 96% already, so that there is not much room left to improve the luminous efficiency by raising the transmittance of the alumina arc tube.
Theoretically, the luminous efficiency may be improved by raising the vapour pressure, i.e., by raising the wall loading of the arc tube, as confirmed by experiments. In practice, however, when the wall loading exceeds the currently used level such as 20 W/cm.sup.2 in the case of the high-pressure sodium vapour discharge lamp, the temperature at the center portion of the arc tube becomes very high, e.g., 1,200.degree. to 1,300.degree. C., so that the metal vapour in the arc tube such as sodium vapour reacts with the alumina tube resulting in the blackening phenomenon which shortens the service life of the discharge lamp. Accordingly, the improvement of the luminous efficiency by raising the wall loading of the arc tube is not practicable.